1. Field of the Invention
The present invention relates to a process for recovering sodium carbonate or other sodium based chemicals from solutions containing sodium bicarbonate and especially sodium bicarbonate solutions obtained from solution mining trona deposits.
2. State of the Art
Sodium carbonate (soda ash) is approximately the eleventh highest volume chemical produced in the United States. It is used in the manufacture of glass, chemicals, soaps and detergents, and aluminum. It is also used in textile processing, petroleum refining, and water treatment.
For many years, sodium carbonate was produced by the Solvay process in which carbon dioxide was dissolved in water containing ammonia (NH.sub.3) and salt (NaCl) to precipitate sodium bicarbonate which was then separated by filtration and heated to form sodium carbonate. Because of high energy costs and problems with disposing of chloride-containing waste streams generated by the Solvay process, it has been abandoned in the United States in favor of obtaining sodium carbonate from naturally occurring trona deposits. Trona deposits are located in Utah, California, and Wyoming. Green River, Wyo. contains the largest known trona deposits in the United States and is actively mined by five companies.
Crude trona ("trona ore") consists primarily (80-95 percent) of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O) and in lesser amounts, sodium chloride (NaCl), sodium sulfate (Na.sub.2 SO.sub.4), organic matter, and insolubles such as clay and shales. In Wyoming, these deposits are located in 25 separate identified beds or zones ranging from 800 to 2800 feet below the earth's surface and are typically extracted by conventional mining techniques such as the room and pillar and longwall methods. The cost of these conventional mining methods is high, representing as much as 35 percent of the production costs for soda ash. Furthermore, recovering trona by these methods becomes more difficult as the best, most thickly bedded trona deposits are depleted. As a result, recovery of carbonate values from trona has fallen in some cases by as much as 5 to 7 percent. Development of new reserves is expensive, requiring a capital investment of as much as $100 to 150 million in 1995 dollars to sink new mining shafts and to install related equipment.
As its chemical composition indicates, trona ore requires processing in order to recover the sodium carbonate. Most of the sodium carbonate from the Green River deposits is produced from the conventionally mined trona ore via the "monohydrate" process. The "monohydrate" process involves crushing and screening the bulk trona ore which, as noted above, contains both sodium carbonate (Na.sub.2 CO.sub.3) and sodium bicarbonate (NaHCO.sub.3) as well as impurities such as silicates and organic matter. After the trona ore is screened, it is calcined (i.e., heated) at temperatures greater than 150.degree. C. to convert sodium bicarbonate to sodium carbonate. The crude soda ash is dissolved in a recycled liquor which is then clarified and filtered to remove the insoluble solids. The liquor is sometimes carbon treated to remove dissolved organic matter which may cause foaming and color problems in the final product, and is again filtered to remove entrained carbon before going to a monohydrate crystallizer unit, a high temperature evaporator system generally having one or more effects (evaporators), where sodium carbonate monohydrate is crystallized. The resulting slurry is centrifuged, and the separated monohydrate crystals are sent to dryers to produce soda ash. The soluble impurities are recycled with the centrate to the crystallizer where they are further concentrated. To maintain final product quality, it eventually becomes necessary to remove the impurities with a crystallizer purge stream.
The production of sodium carbonate using the combination of conventional mining techniques followed by the monohydrate process is becoming more expensive as the higher quality trona deposits become depleted and labor and energy costs increase. As stated above, recovery of sodium carbonate (usually expressed as tons of sodium carbonate produced per ton of trona ore) has fallen as the higher quality, more readily accessible reserves have been mined. Furthermore, the costs of developing new reserves requires substantial capital investment, as much as $100-150 million in 1995 dollars.
Recognizing the economic and physical limitations of conventional underground mining techniques, various solution mining techniques have been proposed. Solution mining allows the recovery of sodium carbonate from trona deposits without the need for sinking costly mining shafts and employing workers in underground mines. In its simplest form, solution mining comprises injecting water (or an aqueous solution) into a deposit of soluble ore, allowing the solution to dissolve as much ore as possible, pumping the solution to the surface, and recovering the dissolved ore from the solution.
For example, a solution mining technique was proposed in U.S. Pat. No. 2,388,009 to Pike on Oct. 30, 1945. Pike discloses a method of producing soda ash from underground trona deposits in Wyoming by injecting a heated brine containing substantially more carbonate than bicarbonate which is unsaturated with respect to the trona, withdrawing the solution from the formation, removing organic matter from the solution with an adsorbent, separating the solution from the adsorbent, crystallizing and recovering sodium sesquicarbonate from the solution, calcining the sesquicarbonate to produce soda ash, and re-injecting the mother liquor from the crystallizing step into the formation.
A second patent to Pike, U.S. Pat. No. 2,625,384, discloses another solution mining method which uses water as a solvent under ambient temperatures to extract trona from existing mined sections of the trona deposits. The subsequent solution is recovered from the mine and heated before dissolving additional dry mined trona in it to form a carbonate liquor having more concentrated values of sodium salts which can subsequently be processed into sodium carbonate.
An additional complicating factor in dissolving trona deposits underground is that sodium carbonate and sodium bicarbonate have different solubilities and dissolving rates in water. These incongruent solubilities of sodium carbonate and sodium bicarbonate can cause bicarbonate "blinding" when employing solution mining techniques. Blinding can slow dissolution and may result in leaving behind significant amounts of reserves in the mine. Blinding occurs as the bicarbonate, which has dissolved in the mining solution tends to redeposit out of the solution onto the exposed surface of the ore as the carbonate saturation in the solution increases, thus "blinding" this surface--and its carbonate values--from further dissolution and recovery. Therefore it is anticipated that long term solution mining of a particular deposit may produce brines with lower sodium carbonate values and higher sodium bicarbonate values than those seen initially. This requires that a process be capable of handling the changing brine grade or that incongruent dissolution must be avoided by some means. "Blinding" is an occurrence which has long been recognized as a problem pertaining to solution mining and is described, for example, in numerous U.S. patents.
U.S. Pat. No. 3,184,287 to Gancy discloses a method for preventing incongruent dissolution and bicarbonate blinding in the mine by using an aqueous solution of an alkali, such as sodium hydroxide having a pH greater than sodium carbonate, as a solvent for solution mining. U.S. Pat. No. 3,953,073 to Kube and U.S. Pat. No. 4,401,635 to Frint also disclose solution mining methods using a solvent containing sodium hydroxide. Unfortunately, alkalis such as sodium hydroxide or lime are expensive and adversely affect the economics of these processes.
The concept of avoiding incongruent dissolution using a brine containing sodium carbonate was discussed by Gancy, supra, and also in U.S. Pat. No. 5,043,149 to Frint, which discloses a specific method to accomplish this. The proposed process disposes of insoluble tailings that remain after uncalcined or calcined trona is dissolved during the process of producing soda ash. The tailings are slurried with water or waste solutions of sodium carbonate or sodium bicarbonate or both and injected into an underground, mined-out cavity. A liquor is removed from the cavity whose concentration of sodium carbonate or sodium bicarbonate or both has been increased and from which sodium-based chemicals may be recovered. Even using these techniques, however, sodium carbonate values can be expected to fall with time and bicarbonate values are likely to be elevated in the brine extracted from the mine.
It is apparent, therefore, that a significant, continuing problem associated with solution mining is the subsequent recovery of the sodium carbonate from the relatively low concentration of carbonate and bicarbonate in the solution mine brine. In recent patents issued to Frint, U.S. Pat. No. 5,262,134, and Copenhafer, U.S. Pat. No. 5,283,054, it is pointed out that past patents such as Kube, No 3,953,073, lack an explanation of how to convert economically these semi-dilute sodium bicarbonate/carbonate mixtures into soda ash. In addition, the solution mining techniques disclosed above produce brines containing sufficient sodium bicarbonate and other impurities to prevent processing into sodium carbonate by the conventional monohydrate process. A major problem experienced is the coprecipitation of sodium sesquicarbonate crystals during sodium carbonate monohydrate crystallization which reduces the quality of the final product.
Several attempts to solve these problems have been made over the years. For example, Miller in U.S. Pat. No. 3,264,057 (1966), describes a process for solution mining of trona. The process directs an aqueous solution to an underground cavity to dissolve trona. The brine removed from underground is split, one part of it going to a steam stripper which converts some bicarbonate to carbonate. The carbonate rich brine from the stripper is recycled to the mine. The other part of the brine stream is directed to an evaporator system (three stages) wherein the brine is concentrated and carbonate crystals are formed by evaporation of water. The final evaporator is operated at 145.degree. C.
Water and some CO.sub.2 are driven off in each evaporation stage, but especially in the final stage. Operation of the final evaporator at 145.degree. C., with the concentration of Na.sub.2 CO.sub.3 increasing and the concentration of NaHCO.sub.3 decreasing, results in anhydrous crystals of Na.sub.2 CO.sub.3, being formed, according to the patent. These crystals are separated by a centrifuge and the wet crystals dried to produce soda ash.
This process requires that enormous quantities of water be circulated through the stripper and mine to keep the alkalinity of the solution mining brine at high levels. The low alkalinity values which would otherwise result would be economically devastating to a process such as Miller's which relies upon evaporation to concentrate the carbonate values and cause crystallization. Also, because many trona deposits are fairly deep, this recirculating load represents an additional economic penalty. This process suffers from other problems, as well, such as the difficulty of producing appropriately sized anhydrous soda ash directly. In addition, the crystallization, centrifugation, and drying steps must all be performed under pressure and at elevated temperatures to avoid the conversion of the anhydrous salt to monohydrate and the extreme processing problems which result from this.
Frint et al., in U.S. Pat. No. 5,262,134, disclose a process for producing soda ash from a solution mine brine containing sodium carbonate and sodium bicarbonate by heating the brine to between 90.degree. C. and 115.degree. C. to evaporate water, convert sodium bicarbonate to sodium carbonate, and to drive off the resulting carbon dioxide therefrom until the concentration of sodium carbonate and sodium bicarbonate in the brine form a solution that will crystallize sodium sesquicarbonate. The sesquicarbonate is then crystallized and separated, and the mother liquor, which is now depleted in bicarbonate, is subjected to a sodium carbonate decahydrate crystallization step. The sesquicarbonate and decahydrate crystals are then processed into various sodium based chemicals including sodium carbonate. This process suffers from the disadvantage of requiring two crystallization steps producing a different crystal species in each step. In addition, it requires the evaporation of large quantities of water early in the process before liquors suitable for feed to more traditional processes can be produced. A small decline in brine grades would adversely affect the economics of such a process because of the tremendous amount of water which would have to be evaporated to produce brines suitable for the later crystalization steps. In addition, the process involves the production of sodium sesquicarbonate, which subsequently yields a significantly less dense soda ash with less wide spread acceptance in the market. Therefore, this process further exemplifies the challenges in handling dilute brines from solution mining processes.
Another process, similar to Frint's which attempts to deal with processing complex bicarbonate and carbonate containing brines was described by Cunningham in U.S. Pat. No. 2,049,249, although in contrast to Frint, it does not deal with brines produced by solution mining. Bulk trona is dissolved to produce a brine which is processed above 17.degree. C. to precipitate sodium bicarbonate. The solid sodium bicarbonate is separated from the mother liquor, which is diluted before it is directed to a cooling crystallizer operated below 17.degree. C. to crystallize sodium carbonate decahydrate. As with the Frint process, U.S. Pat. No. 5,262,134, this process suffers from the disadvantage of requiring two crystallation steps, each producing a different crystal species. The dilution of the mother liquor before decahydrate crystallization is required to avoid forming additional, contaminating, sodium bicarbonate in the decahydrate crystals, but represents an economic penalty. Although this process does not have to deal with incongruent dissolution, blinding and other problems associated with solution mining, it nevertheless produces two products, which means the process can produce only to the level of demand for the less widely used product, sodium bicarbonate.
Copenhafer et al., U.S. Pat. No. 5,283,054, discloses a method for producing soda ash from a brine produced by solution mining. Sodium carbonate and sodium bicarbonate are contained in the dilute brine, which is heated to about 100.degree. C. to about 140.degree. C. to evaporate water, convert sodium bicarbonate to sodium carbonate, and to drive off resulting carbon dioxide. Such concentrated brine having reduced sodium bicarbonate content is then reacted with a sufficient amount of aqueous sodium hydroxide solution to convert essentially all of the remaining sodium bicarbonate in the brine to sodium carbonate. The resulting concentrated sodium carbonate solution having virtually no bicarbonate is further processed to recover soda ash. This solution mining process has been commercially developed and represents the most recent approach to the problems of solution mining of underground trona deposits. Nonetheless, this process still suffers from several drawbacks, including the large requirements of energy to evaporate water to produce concentrated brines. Because the brines are concentrated, caustic is added to take the bicarbonate levels to near zero to avoid sesquicarbonate precipitation in the decahydrate crystallizer. Sodium sesquicarbonate precipitation results in unacceptably high bicarbonate contamination of the decahydrate product, as well as creating severe operating problems related to the poor crystal habit which inevitably results. The high cost of the sodium hydroxide required in the process (even when the caustic is produced on site from carbonate liquors and lime) represents a significant economic penalty. In addition, the operation of a mechanical vapor recompression system for evaporating large quantities of water early in the process adds a significant cost. The process requires a large capital investment for causticizing equipment, evaporators and an MVR.
Solution mining processes generally require either the recirculation of large quantities of pregnant brine or the evaporation of large quantities of water to produce brines with elevated alkalinity values. Many require the production of two products simultaneously and therefore subject the operator to the significant marketing problems associated with this situation. Still other processes, particularly those being commercially developed, require the use of substantial quantities of expensive neutralizing agents such as caustic soda or lime. Unfortunately, these expensive steps tend to offset the costs saved through solution mining. A process which eliminates or reduces the need for these expensive steps, and which produces one product suitable as a feed stock to existing processes producing a variety of products, would finally allow the realization of the advantages expected with solution mining.
Various differences exist between solution mining processes as applied to underground deposits of sodium bicarbonate ores, e.g., trona, and above-ground processes which treat bulk ore which has been conventionally mined. Bulk trona (sodium sesquicarbonate), for example, may be dissolved in an aqueous solvent at high temperatures which are difficult to achieve underground. This allows a much higher concentration to be achieved. After purification, these liquors may be cooled to recrystallize the sesquicarbonate, which is then calcined and converted to soda ash. Alternatively, the bicarbonate content of the mined ore may be decomposed thermally in a calciner, a process which cannot be performed on ore in situ. The calcined ore can then be dissolved in hot liquors to produce an essentially saturated sodium carbonate liquor. In either case, calcination of dry material is required to convert bicarbonate values to carbonate. Therefore, the production of sodium carbonate and other valuable sodium salts by solution mining has generally been more complicated than the production of these same products from conventionally mined ores.
For example, some solution mining processes have included the addition of conventionally mined ore to the mine brine to increase the sodium content of the brine and, therefore, make the brine processable by techniques applied to conventionally mined ore.
A further complicating factor has been the need to eliminate the relatively higher level of soluble impurities associated with solution mine brines. Recently, this has led to the consideration of producing sodium carbonate decahydrate as an intermediate step in the production of sodium salts, including soda ash. Sodium carbonate decahydrate crystals contain ten waters of hydration, which water, unlike entrained or occluded water, is uncontaminated by other sodium salts or other contaminants. Thus, most recently devised processes have focused on the production of sodium carbonate decahydrate. Generally, crystallization is best carried out using concentrated feed streams. This has led to the development of processes (Copenhafer and Frint, for example) involving evaporation to concentrate the brines before they are cooled to produce sodium carbonate decahydrate. In turn, this has created the need to eliminate nearly all of the bicarbonate. Steam stripping has been employed to reduce bicarbonate levels, but cannot economically lower the concentration of bicarbonate in these concentrated brines to a level adequate to avoid bicarbonate related processing problems in the decahydrate crystallizer. The final reduction in bicarbonate levels has been achieved by crystallization of sesquicarbonate in the case of Frint (1/34), or by the addition of caustic in the case of Copenhafer. Therefore, solution mining processes proposed in the prior art are complex and expensive. The present invention, in contrast, provides a simplified and less expensive process for handling solution mining brines.